This is a co-pending patent application to contemporaneously filed patent application Ser. No. 729,276, now U.S. Pat. No. 4,624,639 issued 11/25/76, for an Adjustable Occlusal Plane Table which is incorporated herein by reference. The present invention relates to orthognathic setups and more particularly to an apparatus and method for use with a dental articulator to adjust dental arch models mounted in the articulator.
Dental surgery has achieved remarkable success with many patients. Although sometimes quite complicated and painful, the results achieved have enormous beneficial functional and aesthetic effects. Patients who formerly had gross malocclusions which distorted their appearance and made even the simple act of chewing food a chore have achieved a normal appearance and normal chewing process through dental surgery.
A common surgical procedure for correcting malocclusions involves repositioning the maxillary (upper jaw) and mandible (lower jaw) with respect to the patient's skull. This is accomplished with the aid of two splints. The first splint is used to set the correct position of the maxillary by reference to the uncorrected position of the mandible. The second splint is used to set the position of the mandible by reference to the corrected maxillary position. The surgical procedure is carried out as follows. First, the maxillary is cut from the supporting bone so that it may be repositioned. The first splint is inserted between the patients teeth and the maxillary is then wired to the bone in its new position while its position is fixed by the splint. The first splint is then removed and the mandible is cut from its supporting bone so that it may be repositioned. The second splint is then inserted between the teeth and the mandible is wired to the bone in its new position.
The two surgical splints are made with the aid of a dental articulator from casts (arch models) made of the patient's teeth. The dental casts for the maxillary and mandible are first mounted relative to one another in a conventional manner in the articulator with respect to the hinge axis of the articulator. That is, the arch models are mounted so as to approximate as closely as possible, the position of the patient's teeth with respect to the hinge axis of the patient. The first splint is made after the maxillary dental cast is repositioned to its desired new position. This new position is chosen after studying X-rays of the patient's skull, in addition to a visual examination of the patient's anatomical characteristics. The first splint is then made by placing a moldable material between the repositioned maxillary dental cast and the mandible dental cast and then allowing it to harden. The first splint is then removed. The mandible dental cast is then repositioned to its desired position and the second splint constructed in an analogous manner.
The prior art apparatus and methods for repositioning each of the dental casts from the their current positions to their desired positions are both time consuming and inaccurate. In one prior art method, a series of grid lines are drawn on one of the dental casts, usually the maxillary. An estimate of the positional changes is then made and a wedge portion of the support of the dental cast cut out to create this positional change. The cast is then glued back together. A quantitative measurement of the extent of position adjustment of the maxillary arch model is obtained by measuring the remaining grid lines on the dental cast. Since the grid lines used to make these cuts are drawn by hand, inaccuracies result. Greater inaccuracies result from the fact that the wedge being cut out is substantially above the plane of occlusion of the arch model. Thus, a position change made to the teeth in this manner usually will also create unintended and undesirable other position changes. Furthermore, fine adjustments in the positioning are difficult to make. In addition, multiple casts are needed when this method is used, so that one set of dental casts can be left unaltered to preserve a record of the original position of the patients teeth.
One attempt to improve on the above prior art method is disclosed in U.S. Pat. No. 4,391,589 to Monfredo, et al. This apparatus eliminated the need to cut and glue the support members, by providing a support system for the dental casts which included translational and rotational means for repositioning the dental casts. This apparatus has three problems. First, an operator cannot conveniently use the apparatus to precisely move one of the dental arches to a predetermined position relative to the other dental arch using the dial settings provided. Although the apparatus has dials which indicate the angle through which the dental cast is rotated, these dials are of little use in making a precise change in the position of the dental arch which is located at the end of the dental cast. When the dental cast is rotated about one of the two rotational axes in the Monfredo, et al. apparatus, the dental arch is translated in at least two directions by an amount which depends both on the angle of rotation indicated by the dial on the apparatus and on the distance from the axis of rotation to the dental arch. This distance varies with different dental set-ups. Consequently, one would have to calibrate each dental set-up to take into account this distance and perform a displacement calculation for each rotational movement. This is not practical. As a result, in practice, the movements must be made visually which introduces inaccuracies.
Second, the axes chosen for rotation in the Monfredo, et al. apparatus make it difficult and time consuming to move the teeth to a predetermined location. Even if the setup is calibrated to the particular dental casts, a rotation about one of the axes provided in this prior art apparatus results in unintended and undesirable translational movements of the teeth in addition to the desired rotational movement. To compensate for these additional movements, additional displacement adjustments must be made. The changes in these multiaxis dial settings to accomplish these adjustments depend on the magnitude of the rotation made as well as the distance from the teeth to the axis of rotation. Hence, these adjustments must either be made by using a calibration procedure requiring calculations to be made for each movement or by making visual adjustments which are inaccurate. In either case, the difficulty and time required to reposition the dental cast is significant.
Third, a record of the final position of the teeth is difficult to construct using the Monfredo, et al. apparatus, since the relationship between the actual movement of the teeth and the dial settings depends upon the dimensions of the dental cast and its position on the support. To construct such a record requires a lengthy calibration procedure for each setup.